In electrorefining (ER) and electrowinning (EW) electrodes are immersed in an electrolyte and an electric current is passed between them. The anode is made positive and the cathode made negative so that an electric current passes through the electrolyte from anode to cathode.
In electrorefining (ER), the metal anode is soluble. That is to say that the metal enters into the electrolyte under the influence of the potential between the anode and cathode. For example, in the electrorefining of copper, the anode is made of copper and the copper enters the electrolyte from the anode. The metal, now in the electrolyte, is transported through or by the electrolyte to the cathode where it is deposited. The cathode may be of the same metal as the metal that is being deposited or it may be of a different metal. For example, in the electrorefining of copper it was at one time common to employ a cathode made of copper. However, a stainless steel cathode is now commonly employed which quickly becomes coated with copper and which from then on essentially performs as a copper cathode. The deposited copper is mechanically removed from the stainless steel cathode and the cathode reused. The copper deposited on the cathode is highly pure. Impurities that were in the anode metal fall out as a solid as the anode is dissolved and may contain useful by-products, for example, gold. Besides copper, metals purified by ER include gold, silver, lead, cobalt, nickel, tin and other metals.
Electrowinning (EW) differs from electrorefining in that the metal sought is imported into the cells and is already contained within the electrolyte. In the example of copper, sulphuric acid is typically employed to dissolve copper from an oxide form of copper ore and the resulting liquor, after concentration, is imported into an electrowinning cell to have the copper extracted. An anode and cathode are immersed in the electrolyte and a current is passed between them, again with the anode being positive and the cathode being negative. In electrowinning, the anode is not soluble but is made of an inert material. Typically a lead alloy anode is used in the case of copper. The cathode may be of the same metal that is being extracted from the electrolyte or it may be of a different material. For example, in the case of copper, copper cathodes may be used although stainless steel cathodes are commonly employed which quickly become coated in copper. Under of the influence of the electric current, the metal to be won leaves the electrolyte solution and is deposited in a very pure form on the cathode. The electrolyte is changed by this process having given up a large proportion of its metal content. Besides copper, metals obtained by electrowinning include lead, gold, silver, zinc, chromium, cobalt, manganese, aluminium and other metals. For some metals, such as aluminium, the electrolyte is a molten material rather than an aqueous solution.
As an example of the voltages and current involved, in copper refining, the cell voltage is generally about 0.3V, the current density is about 300 Amps per square meter and the area of each electrode at present is about 1 meter squared. These figures differ considerably for different metals but the invention applies to the refining and winning of all metals.
The electrical characteristics of ER and EW cells differ. In ER cells the over-potentials at the cathode and anode tend to cancel so that the cell has the characteristic of a resistance which in traditional systems is dominated by the electrolyte resistance. In EW cells the net over-potential is not zero and may well constitute the biggest part of the voltage between the anode and cathode. However, in addition there will be some voltage drop due to electrolyte resistance. These characteristics are illustrated in FIG. 13. FIG. 13 uses, by way of example, values approximately typical of those found in the ER and EW of copper.
FIG. 14 illustrates the origin of the ER line in FIG. 13 which shows the relationship between cathode current and anode-cathode voltage for ER. In ER the over-potential of the anode and cathode cancel so that the characteristics of one cathode and its adjacent anodes (consisting in this example of one cathode and two anodes separated by inter-electrode gaps IEG1 and IEG2) are approximately those of a 0.5 milliohm resistor. This resistor is effectively made up of two 1 m Ohm resistors in parallel, 1 m ohm being the approximate resistance of each of the two IEGs.
FIG. 15a shows an electrical circuit representing the ER situation. The total cathode current divides between the two sides of the cathodes in inverse proportion to the resistance of the inter-electrode gap and various other small resistances. The area of each side of the cathode plate is equal. So the current density on each side of the plates is inversely proportional to the resistance of the IEG (and other smaller contributions to resistance). The resistance of each IEG is roughly proportional to the width of the inter-electrode gap (IEG). If the IEGs are of different width, the total current at each side of the cathode (and hence the current density on each side) will be different.
FIG. 15b shows an electrical circuit representing the EW situation. In FIG. 13 the line marked EW shows the relationship between cathode current and anode-cathode voltage for EW. The arrangement of electrodes is the same as shown in FIG. 14. In FIG. 13 the line for EW is displaced upwards by an amount equal to the net over-potential in a cell which for the EW of copper is about 1.5V. For other metals it can be much larger, even above 3.0V. Hence the total voltage across a cell is equal to the sum of the net over-potential and the voltage due to the passage of current through the electrolyte resistance (as well as some other minor contributions to resistance). The approximate electrical equivalent circuit for EW is shown in FIG. 15b. As before with ER, in EW any inequality in the resistance of the electrolyte in the IEG on each side of the cathode can give rise to an inequality in current density on each side of the cathode unless each IEG is individually driven by a controlled current supply. Similarly, any variation in the net over-potential in each of the IEGs will give rise to unequal current density in the IEGs unless each IEG is individually supplied.
Terminology
In ER and EW the starting point is an anode juxtaposed to a cathode in an electrolyte contained in a tank. But many cathode plates and many anode plates may be used, interleaved, with all the anode plates connected in parallel and all the cathode plates connected in parallel contained within a single tank of electrolyte. Electrically this still looks like a single cell and in the industry it is therefore commonly called a cell.
In the ER and EW industry, “cell” is almost universally used to mean a tank filled with anodes and cathodes in parallel.
In the ER and EW industry, “tank” can mean the same as “cell”, above, or it can mean the vessel alone, depending on the context.
So there is potential for confusion if the number of plates in parallel is not alluded to. The present invention is applicable to a cell consisting of one cathode and one anode and one inter-electrode gap (IEG). Hence at the most basic level the word “cell” can be synonymous with a single IEG. In the following description “cell” is used to mean cooperating electrodes separated by an inter-electrode gap. If both sides of the cathode are to be used for metal deposition, two anodes are required giving two IEGs. For further increase in cathode surface area, more anodes and cathodes must be added and hence more IEGs are added. There are twice as many IEGs as cathodes
Referring first to FIG. 1, a basic cell generally designated 24 is shown consisting of one cathode 1 and one anode 2 and one inter-electrode gap (IEG) 3. The cathode 1 and the anode 2 are immersed in an electrolyte 4 contained in a tank 5.
FIG. 2 shows one cathode 1 and two anodes 2 connected in parallel, the whole arrangement creating two IEGs 3.
In tank houses “tanks” are connected in series. A typical ER tank house might therefore require an electrical supply of the order of 36,000 Amps at 250 Volts.
Problems with the Prior Art Processes
In a typical process a number of anode and cathode plates are interleaved and supplied in parallel from positive and negative bus bars so that each anode-cathode pair of plates is effectively supplied from a common voltage source. This results in a spread of current density in the cells due to differences in the resistance of the cells. These differences arise from a spread in the values of, amongst other things, plate separation, plate internal resistance, resistance of the contact between the plates and the bus bars, alignment and flatness of the plates, state of the plates and electrolyte condition.
The efficiency and speed of the electro-production process can be adversely affected if the current density in the cell is not held within certain limits. The quality of the metal deposited can also be affected by the current density.
Additionally a poorly controlled current density can encourage the growth of metal spikes on the plates which can lead to short circuits between the plates.
Many cells are usually connected in parallel by the parallel connection of all anodes in a tank and the parallel connection of all cathodes in a tank but series-parallel connection or series connection is also possible. Hence the current density in a given cell is affected by the condition of other cells and therefore may depart from the ideal.
Electrodes have to be made and positioned to a high accuracy to ensure uniformity of cell characteristics.
The current density that is ideal for one cell may not be ideal for another cell.
The voltage that is ideal for one cell may not be ideal for other cells.
Electrolyte concentration may vary from time to time changing the characteristic of a given cell dynamically during the electrowinning or electrorefining process.
The current to the cells is conveyed over substantial distances at a high current value. Since losses in a conductor are proportional to the square of the current this process is wasteful of energy.
The voltage applied to each cell can be poorly regulated, particularly when supplied through long, high-current bus bars which are loaded with cells the condition of which is variable.
Contact resistance between the plates and the bus bars can vary substantially resulting in poor control of current through the plates and current density on the plates
In some systems, for example in copper refining, a steel cathode is sometimes used with the resulting copper deposition being stripped off and the plate reused. The steel plates can deteriorate with time and use and therefore experience changes in their internal resistance giving rise to poor control of current through the plates and poor current density control on the plates.
The anode thickness and characteristics change during a crop (i.e. during the electro-production process) and between crops making it difficult to obtain the ideal current density during any particular crop.